1. Field of the Invention
The invention relates to the field of clock systems, and in particular, to synchronizing a time of day clock.
2. Statement of the Problem
In some situations, it may be advantageous to have a clock system that is synchronized with a time standard such as Universal Time Coordinated (UTC). It may also be advantageous for two or more systems to have clock systems that are substantially synchronized. For instance, a clock system for a test apparatus may be synchronized with a clock system of a system under test. One method of synchronizing clock systems is by connecting to a Network Time Protocol (NTP) server. The Network Time Protocol (NTP) is used to synchronize the time of a computer client or server to another server or reference time source. NTP provides client accuracies typically within a millisecond on Local Area Networks (LANs) and up to a few tens of milliseconds on Wide Area Networks (WANs) relative to a primary server synchronized to the UTC. A problem with synchronizing a system with the NTP server is the system needs a connection to the NTP server such as a radio receiver, a satellite receiver, or a modem. The NTP server connection may be expensive and/or impractical to use.
Another method of synchronizing clock systems to a time standard is with a Global Positioning System (GPS). A GPS receiver receives a GPS satellite signal from satellites through a GPS antenna. The GPS satellite signal carries a highly accurate time of day signal on a stabilized frequency. The GPS satellite signal also carries a 1 Hz signal and a 10 MHz signal. The time of day signal, the 1 Hz signal, and the 10 MHz signal are synchronized to the UTC. When the GPS receiver is coupled to a clock system, the clock system synchronizes an internal time of day clock based on the time of day signal, the 1 Hz clock signal, and the 10 MHz clock signal. The 10 MHz signal is the reference frequency from which the time of day clock keeps time. The time of day clock is synchronized to the UTC as long as the GPS receiver provides the 10 MHz signal.
A problem arises when a clock system is in a location where the GPS satellite signal cannot be received on a reliable basis. For instance, the clock system is typically in a structure. In such a case, the GPS antenna is mounted on the outside of the structure where the GPS satellite signal can be received. The mounted GPS antenna requires a cable be run through the structure to the GPS receiver. A problem is that situations may arise where it is not possible or desirable to mount a GPS antenna on the structure, or desirable to run the cable through the structure.
A method for synchronizing a time of day clock of a clock system solves the above problems. Advantageously, the method synchronizes the time of day clock that is located where a reliable satellite signal cannot be received. The time of day clock, when in a structure for instance, can be synchronized to the UTC without having to install an antenna on the outside of the structure or run a cable through the structure.
For this method, a portable satellite timing system is initially positioned at a first location where the portable satellite timing system receives a satellite signal. The satellite signal includes a first time of day signal. The portable satellite timing system calibrates its internal clock based on the first time of day signal. From the internal clock, portable satellite timing system generates a second time of day signal. The portable satellite timing system is then transported to a second location and coupled to the clock system. The satellite signal is not available on a reliable basis at the second location, so the portable satellite timing system maintains the second time of day signal while at the second location. The portable satellite timing system transfers the second time of day signal to the clock system. Concurrently, the clock system receives a communication signal from a communication system. The clock system recovers a clock signal from the communication signal and transfers the clock signal to the clock system. The clock system synchronizes its time of day clock based on the second time of day signal and the clock signal.